The human CD8 antigen has recently been demonstrated to be substantially more complex than the CD8 complex, CD8 beta (Lyt-3 homologue) which was thought previously not to exist in man, b) alternate isoforms of CD8 Beta, differing in the transmembrane and cytoplasmic portions of the molecule and c) differential expression of the CD8 alpha/alpha and CD8 alpha/beta dimers within the CD8+ lymphoid population. The heterogeneity of human CD8 has not been well integrated into present models of T-cell activation and T-cell mediated cytotoxicity, which until now have considered CD9 to be homogeneous with regard to structure, function and distribution. It is the goal of this application to more precisely define the structural heterogeneity of human CD8 and to determine the functional similarities and differences between the alternate forms of human CD8 molecules, in order to better understand the physiology of CD8+ lymphocytes. The goals of the project are therefore: 1. Structural Studies - To define the extent of structural variability which can exist in CD8 molecules expressed on the cell surface of human lymphocytes. To determine the predominant CD8 Beta isoform expressed by human peripheral mononuclear leukocytes and thymocytes. To identify the requirements for human CD8 beta surface expression. To identify and/or develop additional monoclonal antibodies specific for CD8 beta and monoclonal antibodies preferentially recognizing the CD8 alpha/alpha, CD8 alpha/beta, and (potentially) CD8 beta/beta (or CD8 beta/x) dimers. To assess whether CD8 alpha and CD8 beta molecules exhibit preferential association with one another or whether the formation of CD8 alpha/alpha and CD8 alpha/beta dimers are random events. 2. Functional Studies - To define the functional significance of each of the various CD8 dimer combinations at the molecular and cellular levels. To assess whether the CD8beta chain contributes to the interaction of CD8 with p56lck or whether the interaction with p56lck occurs solely through CD8a. To assess whether CD8 alpha/alpha and CD8 alpha/beta molecules posses different functional capacities in the processes to T-cell conjugation, activation, and cytotoxicity. To assess whether T-cells bearing only CD8 alpha/alpha homodimers have different requirements for activation and/or inhibition of function than T -cells which express both CD8 alpha and CD8 beta.